I'm pretty sure most 'sharks' will eat the neons and possibly the guppies. Also, ottos do better in groups of at least 3-5. You could maybe try twig catfish... Angelfish might be nice too!
My adventures in keeping Indostomus crocodilus, the crocodile toothpick fish. AKA armored stickleback, paradox fish, and freshwater pipefish. Care is (to the best of my knowledge) exactly the same as Indostomus paradoxus.